So - how did the US function in the 200 years before the Chevron ruling then? I feel like something is missing. I'm fairly ignorant on this matter.
Esp. since people are screaming that FDA, EPA, etc. all won't be able to function with the overturn. But those agencies were created and functioned before that ruling... again, what am I missing?
You're not missing anything, you're being lied to. All this decision does is put the onus on Congress to decide the limits of the powers it gives to federal agencies.
Chevron deference was a legal principle that courts would defer to - meaning, believe - a federal agency's own interpretation of its powers, rather than looking at the law Congress enacted directly. Now the enabling law takes precedence and if Congress wants to expand an agency's power it has to do the job itself. Agencies functioned before
Chevron by staying within the letter of whatever law delegated power to each agency.
Since
Chevron was never a block against lawsuits (like, for example, qualified immunity actually is), but only a principle courts used to decide them, this won't result in agencies or courts being inundated with suits any more than before. There probably will be a brief glut of lawsuits while issues which were previously decided on
Chevron grounds get overturned, but that'll settle down fast as it's just a matter of revisiting old decisions.